Checksum serialisation can collide

[RasmusSkytte]

Describe the bug
Checksum serialisation can collide. NA values are converted to empty strings before hashing, so NA and "" can hash identically. Delimiter-based concatenation can also collide for values containing delimiters.

To Reproduce
N/A

Expected behaviour
N/A

Screenshots
N/A

Provide output from sessionInfo()
N/A

• I have checked that the bug is not already described in open issues

Additional context
N/A